The present invention relates to an automatic active equalizer for a signal pulse transmission cable, said equalizer including at least one amplifier with at least two amplifying devices intercoupled by a feedback circuit, a predetermined closed loop gain versus frequency function of said amplifier being realized by controlling a variable of said function by means of a gain control signal derived from a gain control output of the equalizer. Such an automatic active equalizer is already known from the published French patent application No. 74.11378 and is intended to compensate the distortion and attenuation to which PCM signals are subjected during their transmission over cables. In this known equalizer the feedback circuit ensures negative feedback and comprises a series resonant circuit shunted by a fixed resistance and connected in series with a variable resistance able to be modified by said gain control signal to vary the amount of negative feedback and to realize the wanted gain versus frequency function. To equalize cables having lengths lying between relatively large limits a relatively large variation of the variable resistance is required to perform a correct equalization. For instance, in the known equalizer the variable resistance which is equal to 1/k of a fixed value is able to be varied from a value for which k = 2 to a value for which k = 22 to realize a corresponding variation of the gain which at the peak frequency is proportional to k. At zero frequency where the gain variation ideally should be zero, i.e. independent from the cable length, it is proportional to k/(1+k) so that a relatively large variation of the factor k required to have a good discrimination between different cable lengths at the peak frequency gives rise to an appreciable gain variation at low frequencies. Indeed, for the above given value of k the gain variation from high to low frequencies is from 2/3 to 22/23, i.e. 3.1 dB.